1. Field of the Invention
The present invention relates to a sensorless motor driver for driving a brushless motor as is used to drive the rotation of the cylinder of a VCR (videocassette recorder), the spindle of a floppy disk drive, or the like.
2. Description of the Prior Art
A sensorless motor driver compares, by the use of comparators, the back electromotive forces appearing in the individual coils provided one for each phase in a three-phase brushless motor with the voltage at the node common to all of those three coils, and thereby produces square-wave signals. These square-wave signals are then used by the subsequent circuits to produce driving signals for three phases. As the transistors provided to supply a current to the brushless motor are turned on or off, noise is generated by the back electromotive forces appearing in the individual coils provided one for each phase in the brushless motor. This noise is generated every time the transistor for each phase is turned on or off, and therefore the comparators, which receive back electromotive forces including such noise, outputs square-wave signals including noise. The timing with which the noise is generated is predicted by monitoring the driving signals, and therefore it is customary to eliminate noise from the square-wave signals by the use of a masking circuit in accordance with a noise masking signal. The noise masking signal is produced by a masking signal producing circuit.
In a conventional sensorless motor driver, the masking signal producing circuit outputs pulses having regular pulse widths so that noise is masked only in the periods corresponding to the widths of those pulses. This method, however, has the following disadvantage. When the motor has just been started, or when the load on the motor becomes heavier, the servo system lets more current flow into the brushless motor. This increases the back electromotive forces appearing in the motor, and thus increases the intensity, and also the duration, of the noise generated when the current-supplying transistors are turned on or off. When the duration of the noise exceeds the length of the noise masking periods in the noise masking signal, it is not possible to eliminate the noise completely. This leads to unstable rotation of the motor. To avoid this, it is possible to attempt complete elimination of the noise by making the noise masking periods longer. However, lengthening the noise masking periods means lengthening the periods in which the noise masking signal is kept at a low level, and this undesirably shortens the detection periods in which the detection of the rotation position of the rotor of the motor is possible.
Thus, when the rotation speed of the motor exceeds a certain level, there remains no such detection period at all, and accordingly the motor driver cannot detect the rotation position and thus cannot drive the motor. In this way, the longer the noise masking periods, the lower the upper limit of the rotation frequency of the motor, and this makes it impossible to drive the brushless motor at a satisfactorily high speed.